1. Field of the Invention
The present invention relates to an exposure apparatus, a pressure control method for the same, and a device manufacturing method.
2. Description of the Related Art
The manufacture of a fine semiconductor element such as a semiconductor memory or logic circuit by the photolithography technique employs a reduction projection exposure apparatus. The reduction projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (or a mask) onto a wafer or the like by a projection optical system.
The minimum dimension (resolution) that the reduction projection exposure apparatus can transfer is proportional to the wavelength of light used for exposure and inversely proportional to the numerical aperture (NA) of the projection optical system. The shorter the wavelength, the higher the resolution. Hence, along with the recent demand for further micropatterning of the semiconductor element, the wavelength of exposure light has been decreasing. More specifically, the wavelength of the exposure light has been decreasing to an ultrahigh-pressure mercury lamp (i-line (with a wavelength of about 365 nm)), a KrF excimer laser (with a wavelength of about 248 nm), and an ArF excimer laser (with a wavelength of about 193 nm) in this order.
The feature size of semiconductor elements is shrinking rapidly, while the photolithography technique using ultraviolet-range exposure light as described above has its limitations. To transfer a very fine circuit pattern of 0.1 μm or less efficiently, a reduction projection exposure apparatus (to be referred to as an “EUV exposure apparatus” hereinafter) using extreme ultraviolet (EUV) light having a wavelength of about 10 nm to 15 nm, shorter than that of ultraviolet light, has been developed.
As an EUV light source in the EUV exposure apparatus, for example, a laser plasma source is used. The laser plasma source irradiates a target member placed in a vacuum chamber with a high-intensity pulse laser beam to generate a high-temperature plasma. The EUV light with a wavelength of about, e.g., 13 nm, which is emitted by the plasma is used as the exposure light. As the target member, a thin metal film, an inert gas, a droplet, or the like is used. The target member is supplied into the vacuum chamber by a mechanism such as a gas jet. To increase the average intensity of the EUV light emitted from the target, the higher the repetition frequency of the pulse laser, the better. Usually, the pulse laser is operated with a repetition frequency of several kHz.
In the wavelength region of the EUV light, light absorption by a substance is very large. A refractive optical system utilizing inflection of light, which is used with visible light or ultraviolet light, is not practical, because its EUV light transmittance with respect to an optical element such as a lens is low, and accordingly a reflective optical system is used. As the reticle, a reflective reticle is used, on which a pattern to be transferred is formed by an absorbent arranged on a mirror.
EUV light is largely absorbed by gases. For example, assume an environment where molecules containing carbon such as hydrocarbon remain in a region filled with 10-Pa air. In this environment, light irradiation causes carbon to gradually attach to the surface of an optical member. Carbon absorbs EUV light to decrease the reflectance of the optical member. To prevent carbon from attaching to the surface of the optical member, the region where the optical member to be irradiated with EUV light is arranged must be maintained at a pressure of at least 10−4 Pa or less and preferably 10−6 Pa or less.
In the exposure apparatus, the operation of loading a wafer coated with a resist as a photosensitive agent into the exposure apparatus from the outside and unloading the wafer after transferring the circuit pattern of the reticle to the wafer repeats. A wafer stage has driving mechanisms such as a moving mechanism for scanning and exposure and a transport mechanism which transports the wafer, and accordingly has a very large surface area. These mechanisms generate gases. It is, therefore, very difficult to maintain a wafer stage space within such a low pressure range as mentioned previously.
Furthermore, the resist applied on the wafer is an organic substance, although it is heated and baked before exposure. Thus, when the resist is loaded into the vacuum, the organic substance that forms the resist, a carbon compound resulting from the organic substance by decomposition, and the like vaporize. The vaporized substance diffuses in the exposure apparatus maintained at the vacuum. As the wafer is loaded into the exposure apparatus from the outside, it is difficult to remove air components such as moisture attaching to the wafer within a short period of time before loading the wafer, and the air components are released gradually in the vacuum. Such gases (to be referred to as an outgas hereinafter) emitted from the wafer, the resist, and the constituent components of the exposure apparatus cause contaminants such as carbon to attach to the surface of the optical member. Then, the optical member cannot obtain desired optical characteristics. It may be possible to enhance the vacuum state by using a large-capacity exhaust pump or the like. In this case, carbon-containing molecules or moisture must be prevented from scattering in the exposure apparatus, particularly to the region where the mirror or reticle, the reflectance of which may decrease as carbide attaches, is arranged.
In view of this, a partition may be provided between the wafer stage space and the projection optical system space. An opening may be formed in only a portion of the partition corresponding to the light path where EUV light for exposure passes, to prevent diffusion of the outgas from the wafer stage space into a projection optical system space. Similarly, a partition having an opening in only a light path can be provided between the projection optical system space and a reticle stage space, and between the reticle stage space and an illumination optical system space. To suppress diffusion of the outgas, the opening that connects the two spaces must be formed in the vicinity of the wafer where the EUV light focuses, and also be as small as possible.
Merely forming an opening in the partition may not sufficiently allow the opening to have a desired conductance. A wall having a sufficiently thick opening so as not to block the EUV light path can be set. If the thickness of the opening part is variable, the conductance can be regulated to suit the situation. As the mirror of the projection optical system suppresses optical aberration, the EUV light desirably enters the mirror almost perpendicularly. The mirror that constitutes the optical system also must be arranged close to the wafer or reticle, further adding limitations on the shape of the opening.
To prevent diffusion of the outgas to the projection optical system space, a high-purity inert gas may be supplied. This increases the pressure in the projection optical system space to be higher than that in the wafer stage space, thereby preventing diffusion and inflow of the outgas (see Japanese Patent Laid-Open No. 2005-57154 corresponding to US Publication No. 2005-0030504 A1). As the inert gas, high-purity helium gas, high-purity nitrogen gas, or the like is used. Supply of high-purity helium does not largely attenuate the EUV light in light path space from the light source to the wafer. When compared to a case wherein the entire light path space is maintained at a high vacuum, the attenuation merely changes by as little as several % or less.
When using a gas plasma source as the light source, a filter that selectively transmits only EUV light can be used in the light path, so the gas components of the source will not influence the illumination optical system. As the filter, for example, a Zr filter may be used. The transmittance, however, is about 50% with a 0.5-mm thick filter, and decreases greatly.
In the exposure apparatus, when performing periodic maintenance or parts exchange, vacuum in the space must be broken, and the space must be opened to the atmosphere. In a general vacuum device, when breaking vacuum in the space, the space to break vacuum and the space not to break vacuum are disconnected. An inert gas or dry air is supplied into the space, and the space is pressurized until reaching the atmospheric pressure, thus opening the space to the atmosphere. In the EUV exposure apparatus, however, as the opening and mirror are arranged in the vicinity of the wafer or reticle, it is difficult to provide a mechanism to close the opening. For this reason, when the space is opened to the atmosphere, the wafer stage space communicates with the projection optical system space through the opening, and the gas may shift between the two spaces. In breaking vacuum in a case wherein spaces with different vacuum degrees and outgas amounts coexist through an opening, as in the EUV exposure apparatus, the following problems arise in accordance with the initial pressures in the respective spaces.
(1) When the projection optical system space is maintained at a high vacuum and the wafer stage space has a pressure higher than that in the projection optical system space during exposure, if vacuum of the exposure apparatus is broken, the outgas diffuses and flows to the low-pressure projection optical system space.
(2) When an inert gas is supplied to the projection optical system space during exposure so the projection optical system space has a pressure higher than that in the wafer stage space, when vacuum is broken, the pressure gradient between the projection optical system space and wafer stage system cannot be maintained, as shown in FIG. 13. Then, the outgas may diffuse and flow to the projection optical system space.
If these problems occur, the outgas substance attaches to the surface of the optical element, and desired optical characteristics cannot be obtained. If the outgas diffused in the projection optical system space, degassing from the projection optical system space during re-evacuation takes time to decrease the evacuation efficiency. Such decrease in evacuation efficiency leads to an increase in downtime of the apparatus.
As described above, in the EUV exposure apparatus, an opening formed in the partition between the space where the outgas amount is little and the space where the outgas amount is large makes isolation of the two spaces incomplete. Consequently, the outgas can diffuse from the space where the wafer stage and reticle stage are arranged and the outgas amount is accordingly large, to the space where the projection optical system is arranged or the space where the illumination optical system is arranged, causing a problem.
Hence, in the exposure apparatus having a plurality of vacuum regions which are incompletely isolated from each other, to suppress the outgas from diffusing from the space where the outgas amount is large to the space where the outgas amount is small, when breaking vacuum, is sought for.